Russian President Dmitry Medvedev caused shock and jeers on Wednesday after an obscene insult directed at political opponents appeared on his official Twitter feed.

The Kremlin chief and his more powerful mentor Prime Minister Vladimir Putin have been facing growing opposition to their rule by protesters who say parliamentary elections on Sunday were not fair.

The offensive post appeared to have been retweeted on the MedvedevRussia feed at 33 minutes past midnight, according to cached copies of the feed and a notification of the post received by a Reuters reporter.

"It has become clear that if a person writes the expression 'party of swindlers and thieves' in their blog then they are a stupid sheep getting f****d in the mouth :)" the post read.

The phrase "Party of swindlers and thieves" is used by the opposition to describe Medvedev's ruling United Russia party. It was coined by prominent anti-corruption blogger Alexei Navalny, who was sentenced to 15 days in prison on Tuesday for his role in the anti-Putin protests.

The Kremlin said "an improper retweet" had appeared on Medvedev's account and blamed an unidentified official for having interfered with the feed. "The guilty will be punished," it said in a statement.